1. Field of the Invention
The present invention relates to a method of forming a thin oxysulfide film and more particularly to a method of forming a thin oxysulfide film having a good crystallinity suitable for use as a fluorescent film in thin film EL (electroluminescence) devices and CRTs.
2. Description of the Related Art
Because of many problems in respect of luminance, a dispersive type EL device which uses zinc sulfide (ZnS) fluorescent powder has been deterred its development as a light source for illumination. In its place, a thin film EL device using a thin film phosphor layer has drawn attention recently because it can generate high luminance.
In the thin film EL device, since the luminescent layer is made of a thin transparent film, halation and oozing due to scattering of light incident into the luminescent layer and light generated within the luminescent layer do not occur to any great degree, the thin film EL device exhibits clear and high-contrast display performances. Therefore, the thin film EL device is suitable for display units of vehicle-mounted type and for computer terminals, etc. as well as a light source for illumination.
The thin film EL device is generally of a layered structure comprising a transparent substrate, a transparent electrode made of a tin oxide (SnO.sub.2) layer, a first dielectric layer, a luminescent layer made of a host material layer with luminescent center impurities being added thereto, a second dielectric layer, and a rear electrode made of an aluminum layer, sequentially laminated in this order.
The luminescence process of the thin film EL device is as follows. When a required voltage is applied across the transparent and rear electrodes, an electric field is created within the luminescent layer by which electrons trapped in the interface state are drawn out and accelerated to have sufficient energy and collide with orbital electrons of the luminescent center substance, for example, Eu, to excite the orbital electrons. When the excited luminescent center substance returns to its ground state, it emits light.
In a conventional thin film EL device, a luminescent layer comprising, for example, a host material of Y.sub.2 O.sub.2 S containing Eu as a luminescent center impurity (hereinafter expressed as Y.sub.2 O.sub.2 S:Eu) is formed by the process of sputtering or electron beam deposition.
In the sputtering process, for example, a sintered pellet made of a mixture of Y.sub.2 O.sub.2 S:Eu fluorescent powder and sulfur is sputtered, thereby to deposit the sputtered mixture on a substrate.
According to the conventional method of forming the luminescent layer, when the substrate temperature and the sulfur density are low, Y.sub.2 O.sub.3 :Eu is produced while when the substrate temperature is increased to about 200.degree.-400.degree. C., Y.sub.2 O.sub.2 S:Eu is produced. The resulting Y.sub.2 O.sub.2 S:Eu, however, exhibits a low orientation characteristic and has a granular multi-crystalline structure or a structure containing a so-called dead layer in which many small crystalline grains is produced at the early stage of growth. When the substrate temperature is further increased, orientation characteristics are improved, but sulfur is eliminated, thereby producing Y.sub.2 O.sub.3 :Eu, the undesirable.
When a luminescent layer contains a dead layer, electrons in the luminescent layer accelerated by an externally applied electric field are scattered by a crystalline granular interface before they collide with luminescent center impurities so as to emit light. Thus, the externally applied electric field does not contribute effectively to the light emission.
For a CRT display, Y.sub.2 O.sub.2 S:Eu is used which is produced by sintering for several hours at a temperature of about 1000.degree. C. However, crystallinity is low unless it is sintered at high temperature and the resulting grain size is large, for example, larger than several .mu.m. Therefore, it is disadvantageous when used as a thin film luminescent material.
This problem applies not only to Y.sub.2 O.sub.2 S:Eu, but also to other thin oxysulfide films.
As described above, in the conventional method, a film having high crystallinity cannot be obtained at low temperature while if the substrate temperature is increased, sulfur is eliminated, so that a thin excellent oxysulfide film cannot be obtained in this manner either.